I have a combi boiler I am getting central heating but no hot water to shower it starts off hot to warm to col?

was your shower working before with your combi .if the answer is no then the shower may not be combi compatible.find this out by going on the web site of your shower manufacturer..if the answer is yes then run the bath if this runs hot all the time then your shower is most likely to be the problem and a good plumber will be able to sort this problem (they are cheaper than a gas safe engineer) .if it don't the the combi may have a problem with the diverter valve .to check this turn off the heating and run only the hot water if the pipes to to the heating is hot then the diverter valve is faulty .if heating remains cold then your plate heat exchanger is faulty and for this you need a gas safe engineer .hope this helps